Measured time is dependant on TimerObservers' actions

[Thomsch]

The current implementation of the timer calls directly its observers. For our usecase, the overhead of the calls on the work duration is negligeable. But, if an observer wanted to do a blocking call, it would also block the timer which is not wanted.
A consumer thread should be added to handle the calls to the observers. The timer would then only post updates in a safe way to the consumer thread. Preventing it to be blocked.